Deficiency of argininosuccinate lyase (ASL), the enzyme that cleaves argininosuccinic acid to produce arginine and fumarate in the fourth step of the urea cycle, is characterized by a severe neonatal onset form and a late onset form.

The severe neonatal onset form, which is indistinguishable from that of other urea cycle disorders, is characterized by hyperammonemia within the first few days after birth accompanied by vomiting, lethargy, hypothermia, and poor feeding. In the absence of treatment, lethargy, seizures, and coma worsen, resulting in death.

In contrast, the late onset form ranges from episodic hyperammonemia triggered by acute infection or stress to cognitive impairment, behavioral abnormalities, and/or learning disabilities in the absence of any documented episodes of hyperammonemia.

Both forms of ASL deficiency are caused by recessively inherited mutations in the ASL gene.

Note: Many hospitals include ASL deficiency tests in their newborn screening program.